Direct coupled amplifier for small currents



Sept. 19, 1961 R. A. DANDL 3,001,144

DIRECT COUPLED AMPLIFIER FOR SMALL CURRENTS Filed April 20, 1960 3Sheets-Sheet 1 I (omperes) -10% I l l I I l I l l l (omperes) fig. 1.

INVENTOR. Raphael A. Band! BY ATTORNEY p 9, 1961 R. A. DANDL 3,001,144

DIRECT COUPLED AMPLIFIER FOR SMALL CURRENTS Filed April 20. 1960 3SheetsSheet 2 2 5 1 l I 1 l l I I l I l 10 i 5 2 5 2 5 2 5 2 5 2 5 1o 1o10* 1o 10 1o" 10' I (omperes) '1 Fig. 2.

INVENTOR. Raphael A. Dand/ BY ATTORNEY P 19, 1961 R. A. DANDL 3,001,144

DIRECT COUPLED AMPLIFIER FOR SMALL CURRENTS Filed April 20, 1960 3Sheets-Sheet 3 +!'l*+l'l +I HM All l\]T11 F I EH3 a I; INVENTOR.

% Raphael A. Dand/ BY te tae This invention relates to the detection,amplification, and measurement of very small currents with transistors.

No satisfactory transistor instruments other than those which utilize achopperD.C. to AC. converter-are known to exist for detection andmeasurement of very small currents; i.e., those below about amperes.Such small currents are generated in nuclear radiation counters such asthe surface barrier counter, for example, and in Faraday cup typedetectors for measuring very low currents of particles from particleaccelerators or from plasmas from which the particles escape. Thecurrents are now measured with vibrating reed or vacuum tubeelectrometers. It has heretofore been believed that no suitabletransistorized instrument could be designed because of the extremely lowcurrent amplification (B) characteristics of transistors at such lowinput (base) currents. See Proceedings I.R.E., June 1954, p. 914, whereB is denoted Et Present instruments are not completely satisfactory.They are complex and expensive, in many cases, and do not have the longlife of transistors. They are also temperature sensitive and areinfluenced by vibration and stray fields.

With a knowledge of the shortcomings of prior current detection andmeasurement in the prior art, it is an object of this invention toprovide a transistorized instrument which can operate on currents belowabout 10' amperes accurately, dependably, and with good stability. Otherobjects of the invention will become apparent from a consideration ofthe following description of a preferred embodiment thereof, when readin conjunction with the attached drawings, wherein:

FIG. 1 is a graph showing the relationship between collector current Iand base current I for a selected number of transistors;

FIG. 2 is a graph showing the current amplification (B) as a function ofbase current I and,

FIG. 3 is a schematic circuit diagram of a preferred device.

Referring now to the figures, characteristic curves are plotted forrepresentative transistors of types 2N336 and 2N338. The collectorvoltage was adjusted to a selected voltage with a potentiometerconnected to a supply voltage, the emitter was grounded, and the basecurrent was varied in increments by changing the circuit resistancebetween the supply voltage and the base from 10 to 10 ohms. Thecollector current was measured directly and plotted as a function of thebase current, FIG. 1. The ratio of the incremental change in collectorcurrent to the corresponding increment of base current Patented Sept.19, 1961 of 5 10 amperes, but diverged rapidly thereafter. For the greatmajority of transistors tested, the ratio declined as expected, as shownwith T T and T but for a few, the ratio rose surprisingly to over 10,000at base currents of S l0 amperes. Below 5 X10" amperes, the plottedcurves of FIG. 2 flatten out, so that the maximum values shown aresubstantially equal to B where I =0.

In FIGS. 1 and 2, transistors T was type 2N338, and the measuredcollector current with the base floating, 1 was 9.0 1() amperes. Thus itis apparent that the curves of FIG. 1 also flatten out at values of Ibelow 5x1O The other transistor types and collector currents, inamperes, with their bases floating were as follows: T -2N3369.4 10*""; T2N336-- .2 10 T., 2N3385 X 10*; T -2N336--1.9- 10 T --2N3384.6 1O Thedata were taken at room temperature (about F.) and with acollector-emitter voltage of +0.5 volt. The transistors tested were allwithin manufacturers specifications at normal operating currents. Thecharacteristics are quite reproducible.

These limiting factors in usefulness of transistors for low currnetmeasurements are (1) stability or drift of the transistor currents,which masks variations in input signal; (2) the residual current, orcurrent which flows in the collector circuit with the base floating (Iand which tends to mask the signal; and (3) the current amplification(B). As is noted above, B falls off rapidly for low base currents. Forexample, assume an input signal of 10- amperes is to be detected andmeasured accurately. The collector current of ordinary good transistors,such as T as shown in FIG. 1, is about 2 10- amperes for all values of1,, below 10- amperes. The curves of FIG. 2 shows that an input signalof 10- amperes would produce no difference in collector current from aninput of 10- amperes, or from zero input. Both the slope of the curveand the measured value of I indicate no change from input signals of 0to above 10* amperes. The small changes in I that occur below about 10"amperes 1;, are below 10* amperes, and cannot be detected or measuredwith any degree of accuracy because normal drift of the transistorresulting from normal temperature changes results in collector currentchanges of the order of 10- amperes. It is for just such reasons thattransistors are not used as low current detectors and amplifiers.

I have discovered that some few transistors from types 2N336 and 2N338,upon testing as described above, unexpectedly exhibit a marked increasein current amplification B- for the range of values of I below about l0amperes. It was believed previously that the value of B continue to dropin that range and consequently the transistors were useless. Mydiscovery is dramatically illustrated in the upswing of curves T T and Tand in the greatly reduced slope of curve T T and T of FIG. 2. Whereasfor ordinary good transistors such as T the ratio B is greater thanabout 3 The ratio may be considered as a figure of merit analogous togrid current in an electrometer tube, in that the signal received at thebase will tend to be masked or obscured by m amount proportional to thatratio. A high value of B is desired, with a low residual I The currentthat can be measured with stability is equal to that ratio. Variationsof about times that ratiocan be seen and measured, but not with thedesired degree of confidence in the result. Using transistors of theunique property I have discovered, input currents of the order of 10'amperes can be detected and measured accurately, that is, the figau'e ofmerit is of the order of 10-"; whereas heretofore currents of below 10-amperes were not accurately measurable.

Having demonstrated for the first time that high current amplificationscan be obtained in some transistors at extremely low base currents, andover a considerable range of base currents below 10- amperes, and thatthe residual collector currents I were not so great in those transistorsas to mask such small signals, I have provided a novel amplifier fordetecting, amplifying, and measuring small currents. The first stageutilizes one of these unusual selected transistors, having a figure ofmerit ceo of at least the magnitude of the current to be measured. Thesecond stage preferably uses a selected transistor having an 1 c atleast the reciprocal of the residual current 1 in the first stage. Theremaining transistors need not be selected ones, because the signallevel is great enough so that the unusual small signal response is notrequired.

Referring to FIG. 3', the small current from a detector (currentgenerator) is applied to the base of selected transistor T which is agrounded-emitter amplifier, through input 27. The signal at thecollector is coupled (a) to the base of selected transistor T forfurther amplification and (b) through two phase-ad vance networks 17-18,and 19-20 to the collector of T and the output 21, to stabilize theamplifier at high frequencies. T is an emitter-follower stage having itsoutput signal from its emitter connected to the base of grounded-emitterstage T The output from the collector of T is connected to the emitterof T the base of which is held at a constant potential by means ofdivider network 22, 23. The collector of T is connected to the base of Tso that any current change at the output (collector) of T will betransferred to the base of T even though the respective voltage levelsare significantly different. The base of emitter-follower T is coupledto the collector (output) of T through condenser 24 and diodes 25, 26.Output 21 is connected to the emitter of T and back to the input 27through feedback resistor 28. A standard power supply furnishes :16 voltand ground potentials, so that the output may swing to either :15 volts.In operation, an input signal of either polarity may be received atinput 27. The gain of the amplifier may be adjusted by changing thefeedback resistor 28 to provide the optimum gain for the signals to bemeasured. Feedback resistor values of l0 -l0 ohms are commonly used. Thecollector current to T is first adjusted, through potentiometer 29, tobring the voltage output 21 to zero when no signal is impressed on inputn in) 27. Assume a negative signal current is received, driving the Tbase more negative, reducing the collector current, raising the basepotential of T The potential of the T emitter rises correspondingly asdoes that of the T base. This drops the T collector potential and thatof the T emitter, While the T base potential remains constant, so thatthe T collector drops by an amount corresponding to the emitter drop.Note T is a P-N-P transistor. As the base potential of T drops, lesscollector current flows in T The resulting rise in collector voltage iscoupled to the T base, causing it to draw more current to balance thedecrease in T current. The phase advance networks 17, 18 and 19, 20stabilize the amplifier at high frequencies to prevent oscillation.

Assume a current of 10- amperes is to be measured. A transistor isselected which has a ratio or smaller. If one of the group oftransistors of which T is representative is selected, for example, HG. 3shows that I e E for T12 should be 2.2, 10- or smaller, where 13 is thevalue of B for I =2.2 10- From FIGS. 1 and 2 it is observed that at I=2.2 10- amperes, the ratio for T is ZXIO' lAtX 10 so that T would besatisfactory. T or T would also be satisfactory. For T the ratio is sothat T; is also satisfactory. For the ordinary unselected transistors Tthe ratio is 7 so that T would be generally unsatisfactory, and one ofthe groups T -T should be used for accuracy. The remaining transistorsmay be unselected without. afiecting the accuracy of the amplifier. In apreferred embodiment tested, T was a 2N338 transistor having a B ofgreater than 10 and I of 2X10" amperes, so that the ratio was 2X10- Twas a type 2N336 having a B greater than 300 and an I of 5 10- amperes,with a ratio of 1.7 X 10- Having described my invention, what is claimedas novel is:

1. An amplifier for accurately measuring input currents of magnitudeabout 10- amperes where N is greater than 8, which comprises a pluralityof transistors, each having emitter, collector, and base terminals; aninput terminal; a first, common emitter transistor having its basecoupled to said terminal to receive said input current; a second,emitter-follower transistor having its base coupled to the collector ofsaid first transistor; a third, common emitter transistor having itsbase connected to the emitter of said second transistor; a source ofpotential providing an intermediate reference ground; fourth and fifthtransistors of the same conductivity type, the collector of the fifthtransistor being coupled to the emitter of the fourth transistor and theemitter-collector circuit paths connected in series across said source;an output terminal connected to the emitter of said fourth transistor; avoltage divider network connected between one terminal of said potentialsource and ground; a sixth, coupling transistor complementary to saidthird and fifth transistors having its emitter connected to thecollector of said third transistor, its collector connected to the baseof said fifth transistor, and its base coupled to said voltage dividernetwork; said first transistor having a characteristic ratio I080 of atleast where B is the current amplification for base current of IO-amperes, and I is the collector current with 0.5 volt potentialimpressed between emitter and collector and the base open-circuited;said second transistor having a characteristic ratio 0 of at least 1080where B is the current amplification for base currents equal to l and Iis the collector current; and a feedback resistor connected between saidoutput terminal and the base of said first transistor.

2. A feedback amplifier for small currents of about l0" amperes, Where Nis greater than 8, which comprises an input; a plurality of currentamplifying transistor stages connected in cascade and having an outputstage, at least the first and second stages being provided with selectedfirst and second transistors characterized respectively by ratios B ceo1 1680 where B is the current amplification for base currents of 7-amperes, B is the current amplification at base ourof at least 10 and ofat least rents of amount equal to 1 and I is the collector current with0.5 volt potential impressed between emitter and collector and the baseopen-circuited; a double-ended output circuit comprising third andfourth transistors of the same conductivity type, the collector of thefourth transistor being coupled to the emitter of the third transistor;said coupling being through an intermediate resistance; a feedbackresistor connected between said input and said resistance; a voltagesource; a voltage divider connected across said source; and a couplingtransistor complementary to said fourth transistor and the transistorcomprising said output stage having its base connected to said divider,its emitter input connected to said output stage of said amplifyingstages, and its collector output connected to the base input of saidfourth transistor, the amplifier output being taken at the emitter ofsaid third transistor.

3. In a transistor amplifier, first, second, and third currentamplifying transistors of a first conductivity type connected in cascadeand provided with an input and an output; a source of energizingpotential; an output circuit comprising fourth and fifth transistors ofsaid first conductivity type, each of said transistors having anemitter, a collector, and a base, the collector of said fifth transistorbeing resistance coupled to the emitter of said fourth transistor andthe emitter-collector circuit paths connected across said potentialsource; means connecting the collector of said fifth transistor to thebase of said fourth transistor; means for deriving anoutput signal fromthe emitter of said fourth transistor; a sixth transistor ofcomplementary conductivity type having an emitter connected to saidamplifying stages output, a collector connected to the base of saidfifth transistor, and a base; a voltage di- Aronson et al.:Trausformerless Transistorized A-F Power Amplifier has 7-W Output,August 1958, Instruments and Automation, page 1371.

